Pulse oximetry is a technology that enables the noninvasive monitoring of a patient's arterial blood oxygen saturation and other parameters. The technology was first developed in the 1970s and has been successfully implemented in clinical settings as well as fitness and wellness applications.
In typical prior art pulse oximetry technology, oxygen saturation is measured by means of an optical probe (sensor). The sensor typically includes two light-emitting diodes (LEDs) in the visible (red) and near-infrared regions, and a silicon detector (photodiode) that detects the light emitted by those diodes after it has passed through some part of a patient's body. Typically, the sensor is attached to a blood perfused measurement site (the patient's digit, ear lobe, forehead, etc.), in a reflective or transmissive configuration, to create diffusive light paths through that measurement site. The diffusive light paths typically start at the LEDs and end at the photodiode. Because blood absorbs and scatters light at different rates depending on the applied light wavelength and the blood's oxygen saturation, the red and the near-infrared light wavelengths produced by the LEDs are attenuated at different rates by the site's optical paths (through the patient's body) before reaching the photo diode.
The pulsing of the patient's heart affects measurements with these sensors. For example, the heart's activity during its normal cardiac cycle produces a pulsatile arterial blood flow (plethysmograph) at the measurement site, and that pulsing modulates the light absorbed and scattered by the sensor throughout the site's optical paths. As a result, the red and near-infrared light signals reaching the photodiode also have a “pulsing” pattern, or a pulsatile component (photoplethysmograph), due to the heart/cardiac cycle. Measuring the red and the near-infrared photoplethysmographs over a series of cardiac cycles enables the sensor to noninvasively measure the patient's arterial blood oxygen saturation. This is because the oxygenated blood has higher optical absorption in the near-infrared wavelengths, while the deoxygenated blood has higher optical absorption at the red wavelengths. This property makes it easy to measure the ratio between oxygenated blood and deoxygenated blood (oxygen saturation).
As a consequence, the photoplethysmographs measured by the sensors also typically have fundamental periodicity identical to the heart rate and, therefore, can also be used to measure the patient's heart frequency (pulse rate). The photoplethysmograph intensities are a function of the measurement site's blood perfusion and, typically, the photoplethysmograph associated with the near-infrared wavelength is employed to estimate the site's blood perfusion. The near-infrared wavelength is chosen because it varies less with changes in oxygen saturation, given that the optical properties of oxygenated and deoxygenated blood are less discrepant at the near infrared region. Prior art pulse oximeters typically measure arterial oxygen saturation (SpO2), pulse rate (PR) and the site's blood perfusion (PI).
For clinical or otherwise “critical” applications, acceptable pulse oximeter measurement systems typically require complex electronics and signal processing, which typically requires relatively higher manufacturing costs and power consumption at some point in those systems. This economic and physical reality has a number of implications for the sensors used in such systems and technology.
Prior art pulse oximeters typically use optical sensors that are either reusable or disposable, either wired or wireless, and either “continuous” or “spot-check”, depending on the particular application for which and situation in which the sensor is to be used and other factors. Some prior publications purport to disclose various combinations of those features, but for economic and other reasons, those apparently are not economically feasible, for the market today commonly uses only either (a) wired disposable sensors or (b) wireless reusable sensors. Because of power consumption issues (such as those discussed herein), the wireless sensors typically are only used for “spot-check” applications.
In passing, as discussed herein, “continuous” measurement of a parameter describes a sequence of measurements by a sensor, with sampling that is frequent enough to reliably capture all important parameter trend information of interest. In other words, “continuous” does not mean “absolutely without interruption. “Spot-check” describes when the oximeter measurements are for a generally short period of time, such as during a yearly physical or checkup at a doctor's office.
Each of the two main approaches mentioned above (wired disposable sensors or wireless reusable sensors) involves tradeoffs in costs, functionality, size, comfort, and other factors. For example, although prior art wireless sensors allow a user and/or doctor greater freedom of movement (because there are no wires to get tangled with other things, especially if the patient moves around while wearing the sensor), the power and functionality required to gather and transmit the sensor signals in prior art systems requires the wireless sensor to be relatively large and expensive. The power requirements for “continuous” sensing are also substantial in prior art systems, so typical prior art wireless sensors are only useful for “spot checking” oximeter measurements—the sensors must not only be cleaned between uses but also must be recharged or have their batteries replaced. In other words, the wireless reusable sensors have larger form factor and power consumption requirements, which make them not suitable for continuous use.
Disposable sensors have benefits such as helping reduce the need for cleaning the sensors between uses, but as noted above, because disposable sensors typically are thrown away after a single use, prior art disposable sensors typically are manufactured with relatively little onboard power or processing capability (so as to be less expensive). To achieve those goals, they typically are “wired” to some accompanying host device (because the sensor itself does not include the expensive power supply and complex electronics and required signal processing technology and power capabilities required for wireless sensors). Instead, the required complex electronics and signal processing are typically “offloaded” from the disposable sensors through the wires, to a nearby computer, monitor, or other host device, so that the “work” done by the sensor itself is relatively small. By using wired sensors, the electronics and signal processing can (to at least some degree) be accomplished on that “remote” machine rather than by the sensor assembly itself, and the “disposal” of the sensor does not involve throwing away something of greater cost.
These and other factors have prevented the implementation of a disposable wireless sensor product that can be used for an extended period of time and thus provide a “continuous” set of oximeter data for a patient with the convenience and other benefits of a wireless sensor, while still meeting the quality requirements for clinical and other critical data collection and monitoring.
Others have attempted hybrid systems such as the one in US 2014/0200420 A1, which uses a conventional disposable sensor (the lower cost component) wired to a sensor interface box (the higher cost component) wrapped around the subject's wrist that sends waveforms and or measurements wirelessly to a monitor so that measurements derived by the monitor are generally equivalent to measurements derived by the sensor. For the same reasons aforementioned, and because of the additional sensor wires and connectors required, such topologies have in general a relatively larger footprint, with excessive weight and required body area, making them not practical for continuous monitoring where the patient's comfort is a concern. In addition, the sensor interface box and cables are reusable and thus increase the risk of patient cross-contamination.
For applications that require continuous monitoring, the disposable sensor is preferred for a number of reasons, including:                (i) Lighter and smaller size. The plastic enclosure and mechanical components in reusable sensors typically must be somewhat more rugged to withstand multiple uses. For disposable versions, those components typically are replaced with single-use adhesive tapes.        (ii) More consistent sensor placement and compliance over time, given that the disposable sensor typically is adhered to the patient's measurement site and is less likely to shift from that location.        (iii) Less risk of contamination, given that the sensor is used in a single patient before being disposed.        (iv) Performs better during the patient's physical activity (motion), given that sensor is relatively firmly attached to the patient and the relative motion between sensor and patient's measurement site is minimized.        
As mentioned above, in prior art systems the disposable sensors typically are hard-wired to the monitor by means of a reusable patient cable, to reduce sensor costs and increase reliability. Reusable cables between the sensor and monitor provide other benefits, such as enabling the connection of several disposable sensor models into a single monitor model by simply interchanging connection cables. Prior art connection cables commonly offer and/or use different types of hardware connections and also can be used to configure the behavior of a monitor for a particular sensor application.
Prior art disposable sensor technologies continue to have some risks and limitations, including (by way of example and not by way of limitation):                (i) The prior art connection cables used with the disposable sensors are relatively expensive, and therefore typically are reused. This typically requires that the cables be sterilized before they can be reused, especially in surgery rooms and areas where the risk of infections and contaminations are a concern.        (ii) In a hospital environment (and perhaps most environments), typically it is desirable to simplify the workflow. In that regard, management and sterilization of reusable patient cables adds complications and extra actions and measures to the workflow (rather than simplifying workflow). This in turn increases operating costs and can make the hospital staff and clinicians more prone to errors, including ones that may have a catastrophic effect on patients' safety, recovery, and prognosis.        (iii) The patient's mobility is reduced by use of the cables (since the disposable sensor typically is attached to a monitor by means of a patient cable).        (iv) For applications (such as sleep monitoring) where the subject's comfort is a very important aspect of the procedure outcome, using a cable (typically attached to the disposable sensor) limits the patient's mobility (while at sleep), causing discomfort and potential changes in the subject's sleeping patterns, and thus interfering with the accuracy and/or the monitoring itself.        (v) In the monitoring of patients with highly contagious diseases (such as Ebola, SARS, etc.), the need for a patient cable and monitor nearby the patient increases the chances of cross contamination.        
Other factors affect and/or result in the foregoing and other limitations of prior art disposable sensor technology. As indicated above, clinical grade pulse oximeter system typically requires advanced instrumentation electronics combined with powerful digital signal processors (DSPs) in order to measure SpO2, PR and PI (especially under extreme cases, such as where blood perfusion is low and/or motion and/or physical activity is present or accentuated). In addition, ambient light interferences (such as those caused by exposure of the sensor photodiode to natural light and/or light sources connected to the electric grid) must be filtered out before reliable measurements may be taken. These and/or other requirements can make it difficult to miniaturize the sensor and monitoring technologies, leading to solutions that are relatively more expensive and have higher power consumption, dimensions, and weight.
To minimize customers' recurrent costs, medical device companies typically divide prior art clinical-grade pulse oximeter systems into three main components: (i) a low-cost disposable sensor, (ii) a reusable patient/connection cable, and (iii) a reusable monitor. In such systems, healthcare providers (hospital, clinics, etc.) typically will (a) purchase and reuse the expensive components (the patient cable and monitor) and (b) purchase and throw away after one use the less expensive components (the disposable sensors). The healthcare providers thus must make recurrent purchases of disposable sensors, to be used on new patients and even for subsequent/repeated tests on a single patient. However, this marketing approach has resulted in a relative low volume of sales of monitors and patient/connection cables (when compared to sales of devices in the consumer electronics market, for instance). Combined with the manufacturers' sometimes high operating and development costs in the clinical-grade patient monitoring market segment, the sale prices of those patient cables and monitors can become unaffordable to many or even most healthcare providers. In order to reduce capital expenditure by the healthcare providers and increase sales, medical device companies typically have decided to offer binding contracts for these prior art systems, which enable healthcare providers to obtain monitors and patient cables at a reduced (perhaps loss-leading) price provided that the healthcare providers commit to purchasing the disposable sensors components for a certain period of time (for the entire organization and/or for individual departments (i.e., pediatrics, anesthesia, etc.)). As part of such contracts, the medical device companies also commonly provide training and technical support for the systems. This contractual arrangement typically is referred to in the industry as a full-house conversion.
Even though such disposable sensor supply contract arrangements may be attractive at first (given the relative low initial investment required), the contracts can become expensive over time. In addition to the typical exclusivity clauses in the supply contracts, and even after those periods of exclusivity have expired, the typical hard-wired connections between the sensors and the cable/monitor components allow medical device manufacturers to create physical mechanisms that prevent the healthcare providers from using any competitors' disposable sensors on the manufacturer's proprietary monitors/cables. The proprietary physical cable/monitor/sensor connections can also increase the costs and efforts and risks to healthcare providers if they try to change to a competitors' technology, because they (among other things) have to retrain personnel and change workflow to switch to a new monitoring solution. This puts healthcare providers in a position of very little control over, and relatively few good/flexible/economic options for their patient monitoring needs. It results in relatively higher costs to the healthcare providers and eventually to patients and our healthcare system generally.
Examples of wireless oximeters in the form of a wireless monitoring device which may be connected to a disposable adhesive sensor via a cable connection, are disclosed in U.S. Pat. No. 7,387,607. Other examples of wireless, disposable oximeters are disclosed in U.S. Pat. Nos. 7,486,977, 7,499,739, 8,457,704, and 8,903,467. In these prior art devices, a bandage comprising a single-use, disposable pulse oximeter is self-powered and transmits information wirelessly. In these prior art reflective oximeters, the light emitter and sensor are separated with a foam material. Such foam material may accentuate light piping between the emitter and sensor, and thereby artificially reduce the photoplethysmograph amplitude and accentuate measurement errors due to the position and pressure applied to the measurement site by the adhesive tape. Further, the required larger emitter-detector separation of these prior art oximeters would cause an exponential attenuation of light (not taught in the accompanying disclosures) due to the current and power levels required by both reflective or transmissive oximeters in order to create measurable signals at the sensor/detector with reasonable signal-to-noise ratios. In addition, the small emitter-detector separation necessitated by the required power consumption would create a very shallow penetration depth of the red and near-infrared wavelengths, thus preventing the probing of layers at the measurement site where the pulsatile capillary blood flow modulates the light signals in order to create the photoplethysmographs used to estimate oxygen saturation, pulse rate, perfusion, and their derivative measurements.
The above-mentioned power consumption required to perform both high quality demodulation of optical signals (in order to prevent ambient light interferences) and further estimation of interesting parameters is not accounted for in the disclosed architectures of these prior art wireless oximeters and their host devices. To be feasible, a distributed architecture is necessary where several complex high-latency tasks with floating-point operations are executed by the host device(s). The disclosed prior art oximeters do not include fixed-point low-power and/or low-cost processors that are required for pulse oximeter algorithms of medical grade instrumentation in sensor patches for extended use.
Other prior art oximeters are disclosed in U.S. Pat. No. 8,761,852, which discloses an adhesive, disposable device which transmits data wirelessly and includes a sensor module, a pliable membrane, and a communication module. In this prior art oximeter, optical sensors are connected to a patient by means of a pliable membrane attached to a wristband. U.S. Pat. No. 8,214,007 discloses a patient monitoring device with a plurality of electrical connections to the body of a patient for monitoring the body's electrical activity. This prior art monitoring device suffers from much of the same limitations as the above-mentioned disclosures. Among other things, the emitter-detector separation is not addressed relative to light piping and pressure, and the signal processing and power consumption requirements.